1. Field of the Invention
The invention relates to a semiconductor device and a method of fabricating the same.
This application is a counterpart application of Japanese application Serial Number 313593/2001, filed Oct. 11, 2001, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
There has been growing attention to be paid to Silicon On Insulator (hereinafter referred to as an SOI) during the development of tomorrow's LSIs which can operate at a low voltage with low power consumption. The SOI technique is a technique for forming elements such as transistors on a silicon layer formed on a insulating layer. A semiconductor device fabricated using this technique has an advantage in that it operates at a low voltage with low power consumption. For an application using the current SOI technique, CMOS and the like is mainstream, and hence the SOI technique is not frequently applied to memory products such as a DRAM. However, there is a demand for low voltage with low power consumption even for memories. It has been difficult to fabricate memories which operate at a low voltage with low power consumption compared with current memories using conventional bulk type transistors. Accordingly, the application of the SOI technique to memories has been studied.
A method of fabricating a full depletion type semiconductor device having a conventional SOI structure is described with reference to FIG. 4(A) to FIG. 4(E).
Firstly, an embedded oxide film layer 20 having a thickness on the order of 1000 to 2000 Å is formed on a silicon substrate 10, and a silicon layer 30 having a thickness on the order of 500 to 1000 Å is formed on the embedded oxide film layer 20, as shown in FIG. 4(A).
Secondly, a pad oxide film 35 having a thickness on the order of 50 to 200 Å is formed on the silicon layer 30, and nitride films 36 each having a thickness on the order of 500 to 3000 Å and having openings on a field oxide film formation region are formed on a remaining pad oxide film 35 using a normal photolithography/etching technique, as shown in FIG. 4(B).
Thirdly, the silicon layer 30 is subjected to a thermal oxidation using the nitride films 36 serving as a mask so as to form field oxide films 40 each having a thickness (on the order of 1000 to 2000 Å) to reach the embedded oxide film layer 20. Thereafter, the remaining nitride films 36 and the remaining pad oxide film 35 are respectively removed to form a shape, as shown in FIG. 4(C).
Fourthly, the silicon layer 30 which are separated in elements (hereinafter referred to as elementally separated) is doped with an impurity so as to render the silicon layer 30 formed on the transistor formation region conductive, as shown in FIG. 4(D).
Lastly, a gate oxide film 50 is formed on the silicon layer 30 on the transistor formation region, then gate electrodes 60 are formed on the gate oxide film 50 using a normal photolithography technique and etching technique. Further, the silicon layer 30 is doped with an impurity using the gate electrodes 60 serving as a mask, thereby forming a source and drain regions 71 of the transistor. As a result, the semiconductor device using the SOI technique is accomplished, as shown in FIG. 4(E).
However, if the forgoing transistor is applied to a transfer gate of a memory cell, electric charges are prone to accumulate in a region under a gate electrode of the transistor (hereinafter referred to as a body region). As a result, there is a likelihood that a source and a drain of the transistor are electrically connected to each other by accumulated electric charges even if no voltage is applied to the gate electrode of the transistor. Under such circumstances, electric charges stored in a capacitor connected to the transistor leak, and hence data stored in the memory cell is destroyed.
Although a threshold value (hereinafter referred to as threshold voltage Vt) of a gate voltage of the SOI transistor is of the order of 1.0 to 1.2 volt, there is a possibility that the threshold voltage Vt becomes not more than 0.7 volt owing to electric charges accumulated in a body region of the transistor. Under such circumstances, the source and the drain are electrically connected to each other, as set forth above, and hence there occurs a phenomenon of a pass gate leakage (sub-threshold leakage) where electric charges accumulated in a capacitor leak, resulting in the occurrence of possibility that data stored in the memory cell is destroyed.